In recent years, plastic products with which hard glass fiber is mixed for attaining high strength have increased. In injection molding of such plastic products, wear of a mold is actualized. When the mold wears, surface quality of the products is deteriorated by transfer thereof to the products. The products deteriorated in the surface quality are unmarketable and discarded. It is therefore important that the mold does not wear, and in order to ensure wear resistance, high hardness is required for the mold.
Conventionally, the hardness of the mold used for the injection molding of the plastics with which the hard glass fiber is mixed is mainly from 45 to 55 HRC (from the viewpoint of workability, the mold tempered to a state where the hardness is lower than the above is used in some cases).
In the mold for molding the plastic product, flow passages for temperature adjustment are generally provided in the inside thereof, and cold water, hot water, vapor or the like is allowed to flow through the flow passages to perform temperature control of the mold. However, in the mold with low corrosion resistance, the flow passages are narrowed with rust, and it becomes unable to ensure a predetermined flow rate (the cold water, the hot water, the vapor or the like), resulting in interfering with the temperature control. Further, when the rust is more increased, the flow passages are clogged with the rust, and the flow passages become useless. Furthermore, in the mold with low corrosion resistance, a crack is generated with a rust part as a starting point, and development thereof causes breakage of the mold or leakage of the cold water, the hot water, the vapor or the like from the crack penetrating to a design surface, which sometimes has an adverse influence on the resin product. In addition, a surface of the mold is sometimes corroded by a gas generated from the resin to be molded. When the corroded part is transferred to the product, the surface quality thereof is deteriorated. For such reasons, high corrosion resistance is required for the mold.
Additionally, during use thereof as the mold, thermal stress or mechanical stress is repeatedly applied thereto. In order to avoid breakage thereof under such a severe use environment, fineness of crystal grains is required for the mold.
The mold for plastic injection molding which is required to have the hardness and the corrosion resistance (also including parts constituting a part of the mold) is generally produced through steps of melting→refining→casting→homogenizing heat treatment→hot working→intermediate heat treatment→annealing→machine work 1 (rough machining)→quenching→tempering→machine work 2 (finish machining)→mirror polishing or texturing.
In addition, surface modification (such as PVD, CVD, nitriding, shot blasting or shot peening) is applied in some cases, as needed.
In this production process, (1) no precipitation of grain boundary carbides after the hot working, (2) good annealability and (3) no precipitation of pearlite during the quenching are required for a mold steel.
In the hot working, the steel is in a state of a γ single phase, and all of carbon and carbide forming elements are solid-soluted in a matrix. During cooling after the hot working, the solid solubility of the elements is decreased by a reduction in temperature, and the carbides are sometimes precipitated in γ grain boundaries. The grain boundary carbides precipitated after the hot working cannot be removed by subsequent heat treatment (annealing, quenching or tempering). The grain boundary carbides become foreign matter dispersed in the matrix, which is an obstacle for obtaining a uniform and smooth surface by the mirror polishing. Furthermore, the grain boundary carbides also become starting points of breakage due to repeated stress during use thereof as the mold. Therefore, “(1) difficulty in precipitation of grain boundary carbides” is required.
When the annealability is poor, complicated annealing conditions over a long time are necessary for softening, which causes an increase in material cost. It is therefore required that softening to a state capable of performing the above-mentioned machine work 1 is achieved by simple heat treatment, that is, “(2) good annealability”.
Also pearlite precipitated during the quenching cannot be removed by the subsequent tempering. Pearlite becomes foreign matter dispersed in the matrix, which is an obstacle for obtaining the uniform and smooth surface by the mirror polishing. Furthermore, pearlite also becomes starting point of breakage due to repeated stress during use thereof as the mold. Therefore, “(3) difficulty in precipitation of pearlite” is required.
Conventionally, JIS SUS420J2 has been frequently used in a mold or parts thereof requiring corrosion resistance and a high hardness of about 52 HRC. The components thereof are 0.4% of C, 0.9% of Si, 0.4% of Mn, 0.2% of Ni, 13% of Cr and 0.015% of N. The SUS420J2 satisfies the condition of (2) good annealability described above, and is softened to 87-96 HRB only by simple annealing treatment of cooling it from 850-950° C. to 650° C. at 15-60° C./Hr, followed by natural cooling.
However. SUS420J2 does not satisfy the above-mentioned conditions of (1) and (3).
In particular, even when quench-cooled from a quenching temperature of 1,030° C. at a high rate of 50° C./min, the precipitation of pearlite cannot be avoided.
The quench-cooling rate in the inside of the mold is generally from 10 to 40° C./min (in a temperature range of 550 to 850° C. at which pearlite is precipitated), and therefore, the precipitation of pearlite becomes unavoidable in the inside of the mold of SUS420J2 to increase a risk of breakage during use thereof as the mold.
To the above-mentioned problem, high N stainless steel in which the components of SUS420J2 are largely changed is sometimes used. In this steel, the above-mentioned problem of (1) is avoided by decreasing the C content. The N content is increased, thereby compensating for a decrease in strength due to decreasing the C content. Also, in this steel, the above-mentioned problem of (3) is avoided by increasing the Mn content or the Ni content together with decreasing the C content. However, as a result of such component adjustment, quenchability is excessively increased, and therefore, the above-mentioned condition of (2) cannot be achieved. As a result, cost of the annealing or the machine work 1 (rough machining) is increased, or the time of delivery is forced to be delayed. Further, a γ memory effect is developed during the quenching because of its poor annealability, and coarse grains during the hot working are taken over also during the quenching, resulting in easy generation of cracks during use as the mold.
As described above, the mold for plastic injection molding requires (1) no precipitation of grain boundary carbides after hot working, (2) good annealability and (3) no precipitation of pearlite during quenching, in addition to the high hardness and the high corrosion resistance. However, no mold steel and mold that satisfy these characteristics have hitherto been provided.
The following Patent Documents 1 to 7 disclose steels containing 10.5 to 12.5% of Cr, which is within the range of the present invention. However, all of these steels are not steels for plastic injection molding molds, and different from the present invention in use thereof, as shown below. Furthermore, these steels are different also in essential elements and characteristics under consideration.
Patent Document 1 discloses a free-cutting tool steel having 40 to 47 HRC. However, the steel described in Patent Document 1 is different from the present invention in that it is silent on the plastic injection molding mold with the high hardness and the high corrosion resistance, that S is essentially added for free-cutting, and that the hardness level is lower than that of the present invention. Assuming this steel to be applied to the plastic injection molding mold, it is easily presumed that predetermined mirror finishing properties cannot be ensured due to an influence of the free-cutting component, and that wear resistance thereof is poor.
In addition, an example of containing Cr in a range of 7.05 to 15.0% is not disclosed, and therefore, an effect of containing Cr in the above range is not demonstrated. There is also no attention to the annealability or the precipitation of the grain boundary carbides and pearlite.
Patent Document 2 discloses a free-cutting tool steel having 45 to 63 HRC. However, the steel described in Patent Document 2 is also different from the present invention in that it is silent on the plastic injection molding mold with the high hardness and the high corrosion resistance, that S is essentially added for free-cutting, and that the hardness level is lower than that of the present invention. Assuming this steel to be applied to the plastic injection molding mold, it is easily presumed that predetermined mirror finishing properties cannot be ensured due to an influence of the free-cutting component. There is also no attention to the annealability or the precipitation of the grain boundary carbides and pearlite.
Patent Document 3 discloses an alloy steel for hot working. However, the steel described in Patent Document 3 is silent on the plastic injection molding mold with the high hardness and the high corrosion resistance, and basic components are C, Si, REM and N in some cases. It is therefore easily presumed that quenching is not attained, and moreover, that the corrosion resistance is not obtained. In addition, for Cr as a selective element, an example of containing Cr within a range of 2.5 to 13.0% is not disclosed, and therefore, an effect of containing Cr within the above range is not demonstrated. There is also no attention to the annealability or the precipitation of the grain boundary carbides and pearlite.
Patent Document 4 discloses a steel for a die-casting die having a carbide area ratio of 5.5 to 30% and having excellent erosion resistance. However, the steel described in Patent Document 4 is different from the present invention in that Ni is not essential and is added in an amount of as low as 0.2% (Example), even if added, which does not demonstrate an effect of the high Ni content, and that although Mo+0.5W is essential, it is added in an amount of as large as at least 1.95% (Example), which does not demonstrated an effect of the low Mo content. In addition, an extremely large amount of C is contained because carbides are formed in large amounts. When the steel is applied to the plastic injection molding mold, it is easily presumed that the mirror finishing properties and the corrosion resistance are deteriorated due to an influence of the carbides, and that breakage due to the carbides serving as starting points is generated. There is also no attention to the annealability or the precipitation of the grain boundary carbides and pearlite.
Patent Document 5 discloses a spring steel wire having a diameter of 4.5 to 20 mm. However, the steel wire described in Patent Document 5 is different from the present invention in that it is silent on the plastic injection molding mold, and that V is not essential.
Even when V is selectively added, it is added in an amount of as large as 0.5% (Example), which does not demonstrated an effect of the low V content. Needless to say, the steel wire having a diameter of 4.5 to 20 mm cannot be applied to the mold. There is also no attention to the annealability or the precipitation of the grain boundary carbides and pearlite.
Patent Document 6 and Patent Document 7 disclose oil well stainless steel pipes. The stainless steel pipes described in these Patent Documents are different from the present invention in that these are silent on the plastic injection molding mold, and that Ni, Mo and V are not essential. Furthermore, the content of Si is as low as 0.31% or less (Example), which does not demonstrate an effect of the high Si content. The amount of Ni selectively added is as high as at least 1.63% (Example), which does not demonstrate an effect of the low Ni content. The amount of Mo selectively added is as high as at least 0.75% (Example), which does not demonstrate an effect of the low Mo content. Needless to say, the steel pipes cannot be applied to the mold. There is also no attention to the annealability or the precipitation of the grain boundary carbides and pearlite.
On the other hand, the following Patent Document 8 and Patent Document 9 disclose high Cr steels for plastic injection molding molds. However, in the steels described in these Patent Documents, the amount of Cr added is as high as 12.5% or more, and therefore, the steels are different from the present invention.
In addition, Patent Document 10 discloses a plastic injection molding mold steel which overlaps with the present invention in the amount of Cr added. However, the present invention is directed to the component ranges of Si, Mn and Ni which are not disclosed as Examples in Patent Document 10, and finds effects not obtained by the technique disclosed in this Patent Document.    Patent Document 1: JP-A-57-73171    Patent Document 2: JP-A-57-73172    Patent Document 3: JP-A-58-113352    Patent Document 4: JP-A-2007-197784    Patent Document 5: JP-A-2007-314815    Patent Document 6: JP-A-2008-297602    Patent Document 7: JP-A-2009-167476    Patent Document 8: JP-A-8-253846    Patent Document 9: JP-T-2004-503677    Patent Document 10: JP-T-2010-539325